1. Field of the Invention
The present invention relates to an inverter system, especially to an inverter system with input signal-processing apparatus and method for the same.
2. Description of Prior Art
An inverter is used to convert the direct current (DC) power to the alternating current (AC) power. Therefore, the inverters have been widely applied in solar electricity generating system, motor scooter, uninterruptible power supply, and so on. In an inverter, a voltage/frequency (V/f) control is used to provide variable speed control for motors and consequently inverters can be advantageously applied in refrigeration, air conditioning system, high speed rail system, mass rapid transit system to promote use efficiency of motors so as to increase energy efficiency.
FIG. 1 is a prior art block diagram of an inverter system. A host 10A delivers an analog control signal to an inverter 30A through an analog input/output module 20A. The inverter 30A includes an analog to digital converter (not shown) to convert the analog control signal into a digital signal, which is processed by a microcontroller unit 300A to control the inverter 30A.
At present, the inverters provide control on the frequency of output voltage according to an analog input voltage (such as with range from −10 to 10 volts). However, the analog input voltage has to be processed through a proportional amplifier and then sent into analog input pins of the microcontroller unit 300A. Generally speaking, an operating voltage of the microcontroller unit 300A is +5 volts or +3.3 volts, therefore, the proportional gain is usually a fixed less-than-one value.
FIG. 2 is a prior art circuit diagram for processing an analog input signal of an analog input/output module. The analog input voltage signal VAI (−10V to 10 volts) is proportionally transferred into a voltage −2.5V˜+2.5V through a non-inverting amplifier 22A with a fixed proportional gain P (P=0.25) and the transferred voltage is sent into a DC offset unit 23A, which provides 2.5V DC offset to clamp the transferred voltage to +0˜+5V. The clamped voltage is sent to analog input pins of the microcontroller unit 300A and converted by the analog-to-digital converter unit 24A.
FIG. 3 shows a conversion result performed by the circuit in FIG. 2, wherein an analog input voltage signal is converted into a digital count value with 10-bit resolution. A voltage variation is 5 milli volts (mV) (see formula 1) for a least significant bit (LSB) of the analog-to-digital converter unit 24A when a reference voltage VREF thereof is 5.12 volts. If an error of the analog-to-digital converter unit 24A is ±3 LSB, a digital count value N acquired by the microcontroller unit 300A is 550±3 (see formula 2) when the analog input voltage signal is 1.00 volt.
A width of the LSB is 5 mV:
                                          5.12            ⁢                                                  ⁢                          (              volt              )                                1024                =                  5          ⁢                      (            mV            )                                              (                  formula          ⁢                                          ⁢          1                )            
The digital count value N is calculated:
                    N        =                                            (                                                                    V                    AI                                    ×                  P                                +                2.5                            )                        ×                          1024                              V                REF                                              =                                                    (                                                      1                    ×                                          1                      4                                                        +                  2.5                                )                            ×                              1024                5.12                                      =            550                                              (                  formula          ⁢                                          ⁢          2                )            
Hence, the analog input voltage signal VAI can be inversely calculated from the digital count value N:
                              V          AI                =                                                            (                                  N                  ×                                                            V                      REF                                        /                    1024                                                  )                            -              2.5                        P                    ±                                                    (                                  3                  +                                      1                    ⁢                                          /                                        ⁢                    2                                                  )                            ×                                                V                  REF                                /                1024                                      P                                              (                  formula          ⁢                                          ⁢          3                )            
The analog input voltage signal is 1±0.07 volts when the digital count value N acquired by the microcontroller unit 300A is 550. The error of the microcontroller unit 300A is amplified due to the fixed proportional gain P (P=0.25<1), therefore, a maximum error is 0.07 volts. The error rate increases when the analog input voltage is lower; in other word, the accuracy rate decreases when the analog input voltage is lower so that microcontroller unit 300A cannot be accurately controlled.
Table 1 shows that the accuracy rate of the microcontroller unit 300A gradually decreases when the analog input voltage decreases.
Error rate and accuracy rate of the microcontroller for estimating ananalog input voltage signalAnalog input voltageError rate of theAccuracy rate ofsignal (VAI)microcontrollerthe microcontroller1.00 7.0% 93.0%0.808.75%91.25%0.6011.7% 88.3%0.4017.5% 82.5%0.2035.0% 65.0%0.1070.0% 30.0%